How to cancel Constraint "Track to" for a specific keyframe
Track To constraint has a value slider called Influence, that goes from 0 (no effect) to 1 ( full effect) you can keyframe this as much as any other value in Blender.Selecting the camera, go to Constraints panel, go to the last frame you want the Track To constraint to have effect in, mouseover that value, press i it turns yellow and adds a keyframe, go to the frame you want to elliminate the constraint's effect in, change the value to 0, and repeat the previous operation to add a keyframe.